Linear Variable Filters (LVFs) are dielectric thin film filters similar to more commonly used Dense Wavelength Division Multiplexing filters (DWDM). A typical DWDM filter is a bandpass filter which allows light in a certain wavelength region, or channel, to be transmitted and rejects all other wavelengths within a specified broader wavelength region. These filters are generally etalon or Fabry-Perot cavity based in construction. A dielectric reflective layer is deposited on a substrate followed by a cavity layer and another reflective layer. This forms an etalon where the cavity layer thickness determines the center wavelength of the filter and the reflectivity of the layers determines the filter bandwidth. In order to produce a narrower filter with better out of band rejection, more cavities are added on top of the initial cavity. Of course, in order for multiple cavities to produce a narrower filter the cavity layers should have substantially the same thickness.
Linear variable filters have wedged layers. This has the effect of shifting the filter center wavelength along the wedge axis. As with DWDM filters multiple wedged layers can be stacked on one another.
A LVF can be tuned to different wavelengths by translating the incident beam or the filter in the linearly varying filter direction.
In accordance with the invention LVFs are used to create narrow bandwidth tunable spectral filters.
It is an object of the present invention to provide a linear variable filter having reduced angular and/or beam size broadening thereby minimizing the achievable filter bandwidth.